1. Field of the Invention
The present invention relates to an ink jet printing apparatus and a method for estimating a moisture content of a print sheet. More specifically, the present invention relates to the technique of estimating a moisture content of ink ejected to a print sheet by ink jet printing and reducing deformation (curling or cockling) of the print sheet based on the estimation.
2. Description of the Related Art
Such deformation of print sheets is noticeable in the case of performing high-speed printing to a print sheet that is not a special sheet for ink jet printing, such as commercial offset printing paper, by using aqueous ink. Generally, paper used for the print sheets has properties of expanding when fibers in the paper swell with the paper containing moisture and shrinking when fibers shrink with the paper being dried. This causes curling or cockling of the print sheets due to moisture of ink when ejected to the print sheets.
To solve the above problem, Japanese Patent Laid-open No. 2012-183798 discloses the technique of controlling ejection of ink and drying of ink based on a moisture content measured by an infrared moisture gauge, thereby reducing deformation of print sheets immediately after ink jet printing. Furthermore, Japanese Patent Laid-open No. 2011-51215 discloses, based on the relationship between a moisture content of a print sheet and a change in size of the print sheet, drying the print sheet so that the difference in moisture content between an image portion and a non-image portion of the print sheet immediately before a paper discharging step becomes equal to or smaller than 3.0 g/m2 to reduce cockling. Here, the moisture content is measured by a known method such as a Karl Fischer apparatus.
However, the techniques disclosed in Japanese Patent Laid-open Nos. 2012-183798 and 2011-51215 have a problem that operations and mechanisms for obtaining a moisture content of the print sheet are complicated in the first place, and as a result, productivity of the printing apparatus decreases.
More specifically, the infrared moisture gauge disclosed in Japanese Patent Laid-open No. 2012-183798 requires an operation or processing for obtaining a calibration curve between the reflection intensity of near-infrared rays (IM-D value) and the moisture content for each of color material and concentration of ink used for printing, temperature of a print sheet, and the like. Accordingly, when a new type of print sheet or ink is used, it is necessary to create a calibration curve for each drying temperature, and as a result, productivity of the printing apparatus decreases. Meanwhile, in a case where a trace moisture gauge employing a microwave resonator is used, unlike the method using infrared rays, it is possible to measure a color print sheet without the influence of heat. However, since measurement is made on a dielectric loss factor of water when microwaves pass through the print sheet, it is not easy to measure the dielectric loss factor in a state in which microwaves are passing through the print sheet during a printing operation in which the print sheet is fixed to a sheet holding member, such as a stage, belt, or drum.
On the other hand, the Karl Fischer apparatus or the like using a measurement sample as disclosed in Japanese Patent Laid-open No. 2011-51215 requires creating a sample for each drying temperature when using a new print sheet or ink, and as a result, productivity may decrease.